The present invention relates to filtering apparatus and more particularly to compact porous monolithic filter bodies useful, for example, for the efficient purification of drinking water.
Many filter designs for the removal of unwanted impurities from drinking water supplies are known. These include the increasingly common water carafe filters, commercially sold as replaceable filter cartridges for drinking water carafes at relatively low cost. These filters are reasonably effective for the removal of unwanted tastes, odors, and harmful pollutants from drinking water.
Replaceable filter cartridges of these types are typically packed-bed units containing carbon pellets for the removal of adsorbable/catalyzable constituents such as chlorine and organics. Also included are ion exchange resins for the removal of metal ions such as lead, and in some cases bacteriostatic agents such as silver to inhibit the growth of harmful bacteria.
Although gaining in acceptance, filters designed for use in replaceable cartridge filtration systems suffer from a number of disadvantages. Packed bed systems typically develop preferential flowpaths within the bed, resulting in less than optimal use of the available adsorbents and exchange media and correspondingly decreased purifying effectiveness and service life. Packed beds also tend to release fine adsorbent particles into the treated water during initial use.
Also limiting the utility of these systems is water filtration rate. In general, gravity-assisted water flow rates through these filters are not high. For this reason alone, the use of more tightly packed, higher-efficiency filtration media is not commercially acceptable.
Fluid filtration elements comprising plugged honeycomb filters are well known in the art. U.S. Pat. No. 4,329,162 and published European patent application EP 0745416, for example, describe the use of such filter elements for the removal of particulates from vapor, water and other fluid streams. One disadvantage of these filter elements, however, is cost. The porous ceramic honeycombs used in these filter designs to support carbon adsorbents and provide wall filtration are relatively expensive. Further, manufacturing processes for infiltrating the ceramics with adsorbents and/or other active filtering materials are somewhat elaborate and time-consuming, as are present methods for plugging of the honeycombs to define effective filtration flowpaths through the honeycomb walls.
A number of attempts to decrease the cost and/or increase the efficiency of these honeycomb structures have been made. U.S. Pat. No. 5,006,432, for example, describes a method of making a plugged honeycomb (or "z-flow") filter by first machining a block of open-pore plastic foam to form a filter preform, then infiltrating the preform with a ceramic slurry, and finally heating the filled form to oxidize the foam and sinter the ceramic into a porous honeycomb filter. U.S. Pat. No. 3,788,486 describes pressed or machined porous z-flow filter of similar design for use in filtering molten thermoplastics, while U.S. Pat. No. 2,819,209 describes a pressed fluoropolymer filter for caustic or acidic liquid filtration.
Honeycomb structures for other applications have also been formed by pressing. Thus U.S. Pat. Nos. 2,734,843 and 3,226,460 describe pressing processes and apparatus for the continuous production of lengths of honeycomb core material for the fabrication of reinforced composite structures. These structures are not, however, designed for liquid filtration.
Unfortunately, none of the various alternative methods for making elements of honeycomb structure for use as fluid filters have proven adaptable for the economic commercial production of filters for drinking water purification in large volumes. One particular problem not solved by prior art methods arises from the nature of some of the materials used for the manufacture of these elements. In the case of preforms for activated carbon filters, for example, carbon precursors including phenolic resins or other carbon-yielding organic species can be present which impart a sticky quality to the pressing batch. It is difficult to form thin-walled honeycomb structures from batches containing such precursors, because the thin-walled structures are quite susceptible to damage in the course of separation of the batch material from conventional metal or other forming surfaces.